Simplified selective circuits



Dec- 31, 1935. I H, A. WHEELER 2,02 ,075

SIMPLIFIED SELECTIVE'CIRCUITS Filed April 5, 1933 040/0 Mai/rm .w amm 0.40/0 05mm? RESPOMS/VE r0 1. E

INVENTC JR A HAROLD LWHEELER BY 73...; AM; MM'W ATTORNEYS Patented Dec. 31, 1935 STAT SIMPLIFIED SELECTIVE CIRCUITS Harold A. Wheeler, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware The present invention relates to a radio frequency tuning system for use in connection with radio receiving apparatus of the superheterodyne type.

In accordance with this invention a dual coupling is provided between the antenna and tuned circuits, the characteristics of the two coupling components being so chosen that they will be substantially equal and opposite at the intermediate frequency.

A superheterodyne radio receiver is particularly sensitive to signals of three frequencies, 1. e., the desired signal frequency, the image frequency, which is that frequency displaced above the local oscillator frequency by the same amount that the desired frequency is displaced below the local oscillator frequency, and the intermediate frequency, which is the difference between desired signal and oscillator frequencies. In the past, reception of any except the desired signal frequency has been prevented by the use of a plurality of selective circuits interposed between the signal pickup and the first detector or modulator tube. However, these selective circuits require a plurality of variable tuning elements, which tend to prevent reduction in size and manufacturing costs, while still maintaining a high degree of selectivity.- Arrangements have previously been devised for preventing the reception of image frequency signals without requiring numerous selective circuits. It is the object of this invention to provide a simplified selective circuit requiring but a single tunable circuit andyet preventing not only the reception of image frequency signals but also the reception of signals of the intermediate frequency.

These and further objects of this invention 40 will become apparent from the following specification and claims taken in connection with the accompanying drawing.

In accomplishing the objects of this invention, the modulator or first detector ofa superhetero- 45 dyne radio receiver. is selectively coupled to the antenna through an antenna coupling network including a single tunable circuit. The tunable circuit is coupled to the antenna circuit by means of a dual coupling comprising oppositely varying impedances, the transfer effects of which are equal and opposite at the intermediatefrequency, thus preventing transfer to the tunable circuit of currents of the said intermediate frequency. In addition, the antenna circuit, together with its included impedances, is made resonant at a frequency near the lower end of the band of frequencies over which the coupling is adapted to be operated, for the purpose of increasing the response of the entire circuit at the low frequency end of the band and thus permitting a substantially uniform responsiveness of the network throughout the band. Having thus briefly described the invention, attention is invited to the accompanying drawing, in which: I

Fig. 1 is a schematic diagram of the input circuit to the first detector or modulator of a superheterodyne radio receiver;

Fig. 2 is a schematic diagram showing a modification of the circuit of Fig. 1.

Attention is now particularly invited to Fig. 1 in which the antenna I0 and ground II are connected by an antenna circuit I I consisting of the coil I2, shunted by a small capacitance I3, the lower portion 3-4 of the main tuning inductance. I4, the fixed coupling condenser I5 and the volume control resistor I6. The antenna circuit just described, theinductance of which is principally that of the inductance I2, is, together with its associated capacities made broadly resonant to some frequency near the lower end of the band through which the circuit is adapted to be tuned. For example, in the design of a superheterodyne for use in the broadcast band, the antenna circuit I'I might be made resonant at 600 kc. By properly designing the response characteristic of the antenna circuit,

the responsiveness of the network may be made to vary with frequency in any desired manner, though in the preferred embodiment described more particularly hereinafter, the responsiveness is made substantially uniform over the band of frequencies. The tunable circuit I8 is composed of the coil I4, the coupling condenser I5, and the variable tuning condenser I9, by which the circuit is tuned to respond to the desired signal frequency. The tuning condenser I9 may be connected mechanically to the controlling element of the oscillation frequency circuit for permitting unicontrol in tuning the receiver over a band of frequencies, in a manner well known in the art.

The coupling between the primary or antenna circuit I1 and the tunable secondary circuit I8. is provided by the common inductance 3-4 of coil I4, the mutual inductance between the portion 34 of the coil I4 and the remainder thereof, and by the series coupling condenser I5. The coupling effects of the two coupling elements vary oppositely with frequency, the inductive -by the same reference characters,

Cal

component of the mutual reactance coupling varying directly with the frequency of the signals and the capacitive component of the coupling varying inversely with the frequency. The two coupling components are so chosen that their individual coupling effects between the circuits I1 and I8 will be exactly equal and opposite at the frequency to which the intermediate frequency amplifiers are tuned. For example, in a superheterodyne employing a 450 kc intermediate frequency, these couplings are adjusted so as to give zero resultant coupling effect between these two circuits at 450 kc and thereby reduce interference with the desired signals. The coupling condenser l5 should preferably have a capacitance which is from five to thirty times the maximum of the variable condenser IS, in order to prevent an unnecessary reduction of the tuning range of the circuit l8 as the condenser I9 is varied, while still providing suflicient reactance for satisfactory operation.

The small capacitance l3, together with coil l2, forms a trap to reduce interference at frequencies higher than the tuning range, but does not materially affect the operation of the circuits toward signal or intermediate frequencies.

The grid of the modulator is connected to the tuned circuit l8 by means of the tap 2-on coil H. The tap 2 is so located that the image frequency voltage is at node potential relative .to ground at at least one point in the tuning range of the circuit iii. The position of tap 2 is preferably selected to give best image frequency suppression near the low frequency end ofthe band. The condenser 2| is for the purpose of making the image suppression which results from the tapped coil It more uniform over the entire range and its value is preferably :selected to give best suppression near the high frequency end of the band.

The modulator 20 should preferably be a screen grid tube in order to insure a purely capacitive grid load on the tapped section of the tuned circuit.

The oscillation frequency voltage, which is supplied by an oscillation circuit represented by the oscillation source 221, may be impressed ,in the cathode circuit of, the tube 20 as is usual in 'superheterodyne circuits. The detailsof the oscillation circuit constitute no part of this invention and therefore need not be further described.

The modulator 20, which converts the incoming audio-modulated carrier-frequency signals into audio-modulated intermediate-frequency signals, has its plate connected through the primary 23 of an intermediate-frequency transformer to the direct-voltage source 24. By this means the audio-modulated intermediate-frequency current is supplied to the intermediatefrequency responsive radio receiver 26 and is amphfied therein and reproduced in the usual manner. The screen grid of the tube 20 is supplied with an appropriate voltage by means of the source 24.

The details of the remaining circuits of the receiver are well known in the art and need not be further described.

Attention is now invited to Fig. 2 which represents a modified arrangement. of the input circuits of a superheterodyne radio receiver constructed in accordance with this invention. In this figure, corresponding parts are designated with, however. a prime mark added when the values the intermediate frequency.

M. The condenser 21', for assisting the image 5 frequency suppression, is connected as shown to the junction between the antenna inductance l2 and the primary 25. This same junction is connected through a resistor 26 to ground, the function of this resistor being to reduce interference at frequencies higher than the tuning range. Theremainder of this circuit is, except for the location of tap 3', similar in all respects to that of Fig. 1.

In the arrangement shown in Fig. 2, the in- 15 ductive coupling between the antenna circuit l1 and the tuned circuit I8 includes the mutual inductance between the primary coil and the secondary inductance coil ll, the mutual inductance between part 3'-4 of inductance coil 20 i4 and the remainder thereof, and the selfinductance-of part 3'-4 of inductance coil II. The magnitude of the capacitive coupling provided by the fixed condenser I5 is so chosen,

that it will substantially nullify the coupling or said inductive means, and there will be substantially zero resultant coupling between the antenna circuit l1 and the tuned circuit i8 at In this circuit also 80 the antenna circuit is preferably made broadly resonant to a frequency in the lower part of the tuning range, such as 600 kilocycles in a broadcast receiver. 4 a.

Approximately the following circuit constants v have been found to produce good results in the circuit of Fig. 1, operating over the broadcast range of 550 to 1500 kc, with an intermediate frequency of 450 kc: Resistor IB=5000 ohms.

Coil |2=300 microhenries.

Capacitance l3=4 micro-mid.

Coil H=240 microhenries.

Section 3-4 (of coil Il)=18 microhenries. Mutual inductance, section 3-4 to remainder (of coil I4) =9 microhenries.

Section l-2=12% of turns of coil M.

Condenser 2 I =260 micromfd.

Condenser l5=4700 micro-mid.

Condenser l9 (maximum) =350 micro-mid.

Approximately the following circuit constants have been found to give good results in the circuit of Fig. 2, under similar conditions:

Coil l2=200 microhenries.

Coil 25=100 microhenries.

Resistor 26=10,000 ohms.

Condenser 2 i =8 micro-mid,

Coil [4:240 microhenries.

Section 3'4 (of coil ll) =5 microhenries.

Mutual inductance, section 3'4 to remainder (of coil ll) ='7 microhenries.

Mutual inductance, coil 25 to coil "=15 microhenries.

(Other constants same as for Fig. 1.)

Appropriate cathode heating circuits (not shown) are provided for heating the cathode of the tube 20.

Whereas this invention has been shown and described in connection with a superheterodyne receiver for broadcast signals, it is to be under stood that the invention is equally applicable to receivers or converters operating in any other There is claimed: 1. An antenna 'coupling network operative over a band of frequencies, including in com bination an antenna circuit having connected in series an antenna, a coil and a fixed coupling condenser, a tuned circuit tunable over said band and having connected in series said condenser, a second coil and a variable tuning condenser, and inductance means coupling said circuits in w opposition to the coupling provided by said fixed condenser at frequencies below said band, said antenna circuit being broadly resonant at a fre quency near the lowest frequency of said band, and said fixed condenser having a value between five and thirty. times the maximum capacitance of said variable condenser, said value being proportioned to substantially nullify. the coupling of said inductance means at a fixed frequency somewhat below said band. 2. An antenna coupling network operative over a band of frequencies, including in combination an antenna circuit having connected inseries an antenna, a coil and a fixedcoupling condenser, a tuned circuit tunable over said 5 band and having connected in series said ecupling condenser, a second coil and a variable tuning condenser, and inductance means coupling said circuits in opposition to the coupling provided by said fixed condenser at frequencies below said band, said antenna circuit being broadlyresonani ata frequency near-the lowest frequency of said band, and said fixed-condenser having its magnitude proportioned to substantially nullify the coupling of said inductance 35 Inieais at a fixed frequency somewhat below said an 3. A selective network operative over a band of frequencies, including in combination, pri-' mary and secondary circuits and means for coupling said circuits, said means including at least two predetermined coupling elements whose v coupling effects vary differently with frequency, and whose resultant coupling effect is substantially zero at a particular fixed frequency out- 45 side said band, one of said circuits being tun- -ab1e over said'band, andthe other-of said circuits being broadly. resonant at a frequency near that .edge of said band which is closer to said particular frequency, whereby the responsiveness 'of said network ismade substantially uniform over said band of frequencies.

4 A selective network operative over a band of frequencies, including in combination, primary and secondary circuits and means for coupling said circuits, said means including at least two predetermined 'couplingelements whose coupling efiects vary differently with frequency, and whose resultant coupling effect is substantially zero at a particfularfixed. frequency out- 60 side said band, one of said circuits being tunable oversaid band, and the other of said circuits being broadly resonant at a frequency near that edge of said band which is closer to said particular frequency, whereby the responsiveness of said network is made to varywith fre quency in a predetermined manner. r 5. In a superheterodyne receiver operative oven a band of frequencies and subject to interference at a particular fixed frequency below said band, an arrangement for selectively 1 coupling an antenna to succeeding circuits-of said receiver and for reducing said interference, comprising a A primary circuit including the antenna, a tunable (secondary circuit coupled to said succeeding cir-' 75 cuits, and at least two fixed'coupling elements,

reactive elements is substantially zero.

included in both said primary andsecondary circuits, whose coupling eliects vary oppositely with frequency and whose resultant coupling effect is substantially zero at the said frequency below said band. 5

two fixed coupling elements whose coupling effects vary differently with frequency and whose resultant coupling effect is substantially zero at said frequency outside said band, at least one 20 of said coupling elements being included in said secondary circuit.

. II. In a superheterodyne receiver operative over a band of frequencies and subjectto interference at the intermediate frequency, an antenna coupling network for selectively coupling an antenna to the first detector of the receiver and for reducing said interference, said network comprising a primary circuit adapted to include said 'antenna,- a tunable secondary circuit coupled to the first detector,-and means coupling said primary and secondary circuits, said means including at least two fixed reactive elements individually coupling said primary and secondary circuits and whose coupling effects vary oppositely with frequency; said reactive elements being so proportioned and arranged that: atv

the intermediate frequency the resultant. voltage produced in said secondary circuit by said 8. In a superheterodyne radio receiver operative'over a band of frequencies and subject to interference at intermediate and image frequencies, an antenna coupling network for selectively coupling an antenna to the first de- -tector of the receiver and for reducing said interference, said network comprising a primary circuit adapted to include said antenna, a tunable secondary circuit coupled to said first detector, means coupling said primary and secondary circuits, said means including at least two fixed reactive elements individually coupling said primary and secondary circuits and whose coupling effects vary oppositely with frequency, said reactive elements being so proportioned and arranged that at said intermediate frequency the resultant voltage produced in said secondary circuit by said reactive elements is substantially zero, and means included in said network for suppressing signals of said image frequency. 9. In a superheterodyne receiver operative over a band of frequencies and subject to inter-.- ference at intermediate and image frequencies,

= anantenna coupling network for: selectively coupling an antenna to thefir'st detector of the receiver and for reducing said interference, said network comprising a primary circuit adapted to include said antenna, a tunable secondary circuit coupled to the first detector and means for coupling said primary and,secondary circuits, said means including at least two fixed coupling elements whose coupling effects vary oppositely with frequency and whose resultant coupling-effect is substantially zero at said intermediate frequency, said coupling elements being 76 I included in said secondary circuit, and means in said network for suppressing signals of said image frequency, said primary circuit being broadly resonant at a frequency near the lowest frequency of said band, whereby the responsiveness of said network is made substantially uniform over said band of frequencies.

10. In a superheterodyne receiver operative over a band of frequencies and subject to interference at a particular fixed frequency outside said band, an arrangement for selectively coupling the antenna to succeeding circuits of said receiver and for reducing said interference, com- 11. In a superheterodyne radio receiver operative over a band of frequencies and subject to interference at intermediate and image frequencies, an arrangement for selectively coupling the antenna to succeeding circuits vof said receiver and for reducing said interference, comprising a primary circuit adapted to include said antenna, a tunable secondary circuit coupled to said succeeding circuits and means for coupling said primary and secondary circuits including at least two fixed reactive elements individually coupling said primary and secondary. circuits and the coupling effects of which vary oppositely with frequency, said reactive elements being so proportioned and arranged that at said intermediate frequency the resultant voltage produced in said secondary circuit by said reactive elements is substantially zero, and means in said network for suppressing signals of said image frequency, said 20 

